Who says? Suppose I define the term "mammal" like so:

If someone said "fur must mean something else because everything else in the list is in the class 'things inside the body'" <-- I would think that's a silly statement

In any list of things, you can always find some principle of distinction by which one of its items isn't in the same class as the others. Therefore not every item in a list has to be in all of the same classes as the others. If this was a requirement, definitions could never contain lists while remaining true.